Picture boxes are pretty simple when you get the hang of them - the important properties for you are Image and SizeMode.
The first of these determines which picture will be displayed, and the second says how it it shown.
Going backwards, set the SizeMode to StretchImage - you can do this in the designer via the Properties pane, or in code by:
myPictureBox.SizeMode = PictureBoxSizeMode.StretchImage;
StrechImage mode means that the picture will be re-sized to fit in the PictureBox.
Now, set the Image. Again, you can do this in the designer, via the properties pane, or in code:
myPictureBox.Image = Image.FromFile(@"D:\Temp\MyPic.jpg");
Where the bit in the double quotes is the full path and name of the file. The '@' at the start just says "turn off special character processing" and lets you type the path with single backslash characters.
Run it! It should display the image from the file in your PictureBox control.
[edit]Typos. Lots of the buggers. - OriginalGriff[/edit]